Language Outcomes, Mechanisms, and Trajectories in Adults With and Without Developmental Language Disorder

Aim 1: Characterize individual differences in the developmental trajectory and ultimate attainment of language ability in adults with and without DLD. This proposal directly addresses previous limitations by leveraging existing data and an existing participant pool from one of the most unique, influential, and comprehensive examinations of childhood DLD to date (the Iowa Longitudinal Study). The investigators re-recruit participants with and without DLD from this historic study who are now adults (30-35 years old). The investigators use retrospective oral language measures from kindergarten through 10th grade and collect new measures that capture language comprehensively (receptive and expressive language at the sublexical, lexical, sentence, and discourse/pragmatic levels) in adulthood. The investigators focus on the following two sub-aims:

Aim 1a: Characterize language outcomes in adults with DLD. The proposed measures will capture language abilities that stratify a wide range of language skills in adolescents and adults, differentiate individuals with DLD from their typically developing peers at various stages of development, and are reported to be functional for adults in the workplace. Given documented difficulties with complex language structures and discourse in older children and adolescents with DLD, the investigators expect that adults with DLD may diverge from adults with TL in performance related to language skills that are more linguistically complex (e.g., complex syntax and high lexical density). Further, they predict marked deficits in higher-level language skills (pragmatics and discourse), which are important aspects of communication in the workplace (e.g., language used for negotiating, cooperating, etc.). Given variable profiles of DLD, the investigators also expect that some participants will demonstrate lingering differences related to the consistency of their errors and the phonological complexity of their utterances. The investigators also seek to answer questions about the dimensionality of language ability. This follows from previous work demonstrating that language ability is best captured by a unitary factor in the early school-age years, with vocabulary and grammar becoming differentiated only later in development. Accordingly, the investigators carry out analyses to determine whether this dual-factor approach holds in adulthood or whether the dimensionality has increased yet further, and whether differences in language dimensionality obtain across DLD and TL groups.

Aim 1b: Shape of developmental trajectory. The investigators use retrospective language measures collected from K-10th grade as part of the Iowa Longitudinal Study, combined with new measures the investigators will collect in these same individuals in adulthood to examine the long-term trajectory of DLD. Given that adults with DLD do not typically receive services, the status quo under which the field is implicitly operating is that adults with DLD "catch-up" to their TL peers or that language abilities plateau in the school-age years and no further development is possible. However, research shows that adults with DLD have lower quality of life outcomes, and language abilities continue to grow until the thirties in typical populations, which conflicts with this viewpoint. For this sub-aim, the investigators use latent-trait Bayesian analyses to mediate between two competing hypotheses. The first hypothesis is that the investigators will uncover evidence for the maintenance of the relative stability seen during the school-age years (i.e., children, with remarkable reliability, maintain their standing relative to the rest of the cohort members in longitudinal studies of language ability). However, upon exiting the educational system, adults with DLD rarely receive services, and this may contribute to increased variability in outcomes. Thus, the second, competing hypothesis is that the investigators will uncover a fanning effect: some adults will have improved their language abilities relative to their prior standing in the cohort, while others will show evidence of a decline, with still others showing a relative maintenance of the school-age years trajectory. Such a fanning effect would suggest that trajectories after the school-age years are amenable to intervention.

Aim 2: Identify real-time processes that explain individual differences in language outcomes in adults with and without DLD.

For Aim 2, the investigators offer a mechanistic account of language deficits underlying DLD in adults. Their goal is to refine processing-based theories of DLD by offering a more precise characterization of the computational differences that subserve deficits in language processing in this population. Further, the investigators seek to illuminate individual differences in adult language across the ability spectrum. For this aim, the investigators use well supported models of competition in adults with TL as a theoretical framework for understanding mechanisms that underlie language deficits in adults with DLD. The investigators have developed a research protocol that measures real-time competition in expressive and receptive language at the lexical, sentence, and pragmatic levels. Consistent with working memory accounts, the investigators predict that, compared to adults with TL, adults with DLD will show sustained competition at all levels of language. An alternative-though not mutually exclusive- hypothesis is consistent with speed of processing accounts: group differences will be better reflected in how quickly targets and competitors are initially activated. Further, the investigators predict that measures related to the dynamics of competition will account for variation in language outcomes in adults across the ability spectrum.